Newbie - 1st purchase in over 10 years

I recently purchased a DB recoil comp for $500. I couldn't pass it up. My last bike was a Proflex with just about everything upgraded. I haven't ridden a Mt bike for a while. So hopefully this thing is better than a Wally World bike. A little concerned with the weight. I never like anything to be too heavy, but I also didn't spend $1Ks. If I really get back into it and like the 29er concept; my next purchase will probably be a Giant Trance or a Sortie. I'll be moving to HI next year once I return from Greenland.
My planned style of riding is probably trail/all Mt.

I do have some newbie questions. I recently purchased a Rock Shox Reba silver Tk 120mm (off a 29er Trance). It has a 15mm dropout, so I need to convert my hub, can I do that and if so where/who to purchase the parts from?

Also will the extra 20mm really change the geometry of the bike drastically?

i appreciate it and look forward to getting into the sport/hobby again.

Not all hub/wheel companies are able to convert from a 9mm to 15mm or vice versa. Certain brands like Hope, Easton, Mavic, American Classic, Cole and a few others have kits so that you can run whichever you need. All are pretty easy to switch. Not sure what kind of hubs are on your Recoil, but more than likely, you won't be able to make the conversion unless you buy a whole new hub or you can just purchase a front wheel.

As for the shock. Its usually not recommended to add 20mm more travel to the front head tube, but that really depends on your head tube angle. If you called DB, they'll tell you not to do it, but I know a few people that have done it. 10mm, I wouldn't really worry about it but anything more, you may need to be cautious, especially on those bikes that are suited for intermediates. I have heard of people cracking their head tubes because of the added stress it creates. Aside from the dangers of cracking the head tube, changing the geometry will also effects the way the bike handles and it can be pretty dramatic.

I don't think +/- 20mm is that big a deal unless you care about the warranty. It takes a lot to break a frame, however...

You almost certainly need a whole new front wheel.

OP, in your shoes, I'd try the fork with its full travel first. If I couldn't set the bike up to climb without wandering all over the place, I'd order the spacer to make it a 100mm fork - RS forks have a part available to make this modification.

If your style is more AM, you may not even care about how the bike climbs.